A lanyard would be on my person - and as I am able to speak for myself, I feel I would not wish to wear that personal 'display' - rather, I would voice appropriately when required.
However, your comparative comment concerning a legal driving license - rather like a passport - that is an official ID doc for far-reaching purposes.
As regards displaying a disabled 'explanation' - upon a vehicle not occupied at the time of parking - that is an entirely different matter - cars cannot speak, and such notification on vehicles serves to inform during the vehicle driver's absence.
I have respect for those who do feel the wearing of a lanyard is right for them - as I said, I could only speak for myself and not on behalf of another ..... But I hope in my explanation this helps to clarify why I personally would not wear something on my person - whilst I am in the position of speaking up on my own behalf.
Perhaps a 'flash card' with an updatable 'current' verification system would aid a speedy passage through customs/security?
Doubtless such questions are regularly raised within the security of airports and other installations.
Best wishes
Jayne